In the production of oil from subterranean oil reservoirs by various flooding techniques, especially waterflooding, it has become a common expedient to add various polymeric thickening agents to the water in order to increase its viscosity to a point where it approaches that of the crude oil which is to be displaced so as to improve the displacement of the oil from the reservoir. The use of polymers for this purpose is often stated to be for mobility control.
Another problem which arises in the various flooding processes is that different strata or zones in the reservoir often possess different permeabilities so that displacing fluids enter the high permeability or "thief" zones in preference to zones of lower permeability where significant quantities of oil may be left unless measures are taken to plug the high permeability zones wholly or partly and so divert the displacing fluid into the low permeability zones. Mechanical isolation of the theif zones has been tried but vertical communication among reservoir strata often renders such measures ineffective. Physical plugging of the high permeability zones by cements and solid slurries has also been attempted with varying degrees of success but here, the most serious drawback is the possibility of permanently closing still productive horizons.
From these early experiences, the desirability of designing a viscous slug capable of sealing off the most permeable layers so that the floodwater would be diverted to the underswept, tighter regions of the reservoir, became evident. This led to the use of oil/water emulsions, gels and polymers for controlling the permeability of the formations in a process frequently referred to as a "profile control", a reference to the control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed have included oil/water emulsions, gels, e.g. lignosulfonate gels and polymers, with polymers being the most extensively applied in recent years.
Among the polymers so far examined for improving waterflood conformance are polyacrylamides, polysaccharides, celluloses, furfural-alcohol and acrylic/epoxy resins, silicates and polyisocyanurates. A major part of this work has been conducted with the polyacrylamides.
Polyacrylamides have been used both in their normal, noncrosslinked form as well as in the form of cross-linked metal complexes, as described, for example, in U.S. Pat. Nos. 4,009,755; 4,069,869 and 4,413,680. In either form, the beneficial effects derived from these polyacrylamides seem to dissipate rapidly due to shear degradation during injection and sensitivity to reservoir brines. To overcome these problems and to achieve deeper penetration into the reservoir, dilute solutions of these polymers have sometimes been injected first and then complexed in situ. For example, in one such process, three sequential injection steps are employed: cationic polyacrylamides are injected first for strong adsorption and anchoring onto the generally anionic sites of the reservoir rock surfaces, followed by chelation with aluminum ions provided by aluminum citrate or with chromium ions generated by the in situ reduction of dichromate ions and finally, anionic polyacrylamides are injected for the formation of the desired cationic polymer-metal ion-anionic polymer complexes (J. E. Hassert, and P. D. Flemming, III, "Gelled Polymer Technology for Control of Water in Injection and Production Wells," 3rd Conference on Tertiary Oil Recovery, U. of Kansas, Lawrence, 1979).
Another group of polymeric thickeners which has received considerable attention for use in waterflooding is the polysaccharides, particularly those produced by the action of bacteria of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307 disclose mobility control by the use of polysaccharides in the presence of polyvalent metal ion crosslinking agents. U.S. Pat. No. 3,810,882 discloses the possibility of using certain reducible complex metal ions as cross-linking agents for polysaccharides. U.S. Pat. Nos. 4,078,607 and 4,104,193 describe a method for improving the efficiency of waterflooding operations by a particular polysaccharide prehydration technique. U.S. Pat. No. 4,413,680 describes the use of cross-linked polysaccharides for selective permeability control in oil reservoirs.
U.S. Pat. No. 3,908,760 describes a polymer waterflooding process in which a gelled, water-soluble Xanthomonas polysaccharide is injected into a stratified reservoir to form a slug, band or front of gel extending vertically across both high permeability and low permeability strata. This patent also suggests the use of complexed polysaccharides to block natural or man made fractures in formations. The use of polyvalent metal ions for cross-linking polysaccharides and other polymers including polyacrylamides, which are to be used for permeability control is described in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680.
One problem which has continually attended the use of polymeric mobility and profile control agents is that thickened aqueous solutions, e.g. polysaccharide solutions, may be more difficult to inject into the reservoir than less viscous solutions. Also, the shear conditions encountered during injection may degrade the polymer and reduce its effect when it enters the reservoir. To overcome the injectivity problem, U.S. Pat. No. 3,208,518 proposed the use of polymer solutions of controlled pH which undergo a delayed increase in viscosity after the solution enters the formation and the pH changes by neutralization of acidic or basic constituents in the solution by materials present in the reservoir.
The problem of retaining good gel strength as well as good injectivity has, however, remained. To a great extent, the possession of good gel strength has been considered inimicable to good injectivity because gel strength is dependent upon the possession of a well-developed gel structure whereas injectivity requires that the polymer function move as a fluid, implying the absence of gel structure. Thus, polymer gel selection has remained something of a compromise and no adequate solution to this problem has so far been achieved. To this should be added the problem of shear stability. Although many of those polymers undergo reversible shear thinning, the degradation brought about by shear forces may not be wholly reversible so that after a polymer has been pumped under pressure down the well into a reservoir, it may be far less effective for its intended function than it was prior to injection. Thus, there is a need to develop polymers with good gel strength, injectivity, shear stability and other rheological properties.